Communication over optical fiber is gradually displacing the more traditional electronic or radio-wave transmission. Originally, optical fibers were used predominantly for high-density, long-haul applications in which data-containing electrical signals were used to modulate a laser at one end of the fiber. The light received at the other end of the fiber was detected and converted to an electrical signal in the detection process. Such a process avoided the necessity of switching an optical signal between selected fibers. Switching was predominantly done on the electrical signals derived from the optical signals. If the signal was to be retransmitted on fiber, it was again converted to optical form.
Optical fibers have many advantages over copper cable, whether it be twisted-pair copper cable for local distribution or coaxial cable for high-bandwidth transmission. In the past, however, optical fibers were too expensive to justify their substitution in local distribution networks. Now, their cost is fast approaching that of twisted-pair copper cable. It is expected that local fiber networks will become commonplace, whether the fiber is extended from the central office to the home or office or more restrictedly to a remote distribution node in the neighborhood. Local telephone fibers will carry optical optical signals that need to be switched at the central office.
Therefore, there has been an effort to develop a switch capable of directly switching an optical signal. That is, there is a desire to be able to switch the optical signal from one of M optical input fibers to any one of N optical output fibers without converting that signal to electrical form.
One part of the problem of an optically switching central office or other large scale switching node is the requirement for an optical cross-connect. In the copper-based central office of the past and the fiber-based one of the future, copper cable or fibers emanating from the home are not directly and permanently connected to the switching equipment. Instead, there is a cross-connect acting as an interface between the wire or fiber from the field and the wire or fiber within the office. For copper cable, the cross-connect is called a main distribution frame, in which central office cables are run horizontally and outside cables are run vertically. Jumper cables are installed between the horizontal and vertical sides of the frame to connect the outside copper wire to the appropriate point of the switching equipment during initial hookup of services. Any time a different equipment switching point is required, the jumper cables are manually reconnected.
The advantage of the copper jumper cable is that it provides a semi-permanent but reconfigurable connection, even if it requires manual attachment and removal. It is, however, not seen how optical fiber can be adapted to the distribution frame configuration.
It would be preferred that the optical cross-connect be electronically reconfigurable but be bistable. That is, once the cross connection is connected or disconnected, that configuration is maintained without further application of electrical power. Such bistability would allow the possibility of matrix addressing of hopefully large switching matrices. Existing mechanical optical switches are bistable but lack simple electronic control and, in any case, are too bulky, expensive and unreliable for central office use. Electro-optic devices have been proposed for optical switches. They offer the advantage of being amenable to photolithographic definition of a large number of switching elements on a single substrate. However, electro-optic devices are not bistable.
Additional desired qualities of an optical cross-connect are low loss, polarization independence, wavelength and bit-rate independence and low cross-talk between channels. To date, no technology has offered an electronic solution for the many demands of an optical cross connect.
Lee has disclosed in U.S. Pat. No. 4,888,785 an optical beam splitter. This same work has been reported by Osinski et al in a technical article entitled "Miniature integrated optical beam-splitter in AlGaAs/GaAs ridge waveguides" appearing in Electronics Letters, volume 23, 1987 at pages 1156-1158. This beam splitter consisted of two intersecting semiconductor waveguides. A slot of a precise thickness was cut through the waveguides at the intersection such that some radiation was transmitted across the slot and some was reflected at the slot interface to the intersecting waveguide.